In an internal combustion engine there may be a metal chain, also known as a timing chain, comprised of bearing pins, rollers, bushings, and an inner and outer plate. Due to the significant load and friction exerted on these components, the timing chain is susceptible to significant wear including corrosive wear. To address this problem lubricants are used to reduce wear between moving parts where there is metal to metal contact.
Chain elongation, or timing chain stretch, is a phenomenon that occurs in internal combustion engines with a timing chain that has deteriorated due to wear. Chain elongation mainly occurs at the pin, bushing and side plate wear contact interfaces. Timing chain stretch can lead to significant problems in operation of the internal combustion engine and can have an effect on engine performance, fuel economy and emissions.
Chain elongation can cause a deviation from the desired timing of parts operatively connected to the timing chain. Such a deviation may be caused, for example, by the chain skipping one or more sprocket teeth during operation, or exceeding the adjustability of the cam phasers. These deviations may alter the relative timing of the valves and ignition. Intake valve timing affects when the air and/or fuel mixture is drawn into the cylinder. Exhaust valve timing affects power output as power can be lost as a result of escape of gas via the exhaust valve if the exhaust valve does not open at the appropriate time. Additionally, unburned hydrocarbon emissions can increase when exhaust valve timing is off since unburned combustion gas may escape via the exhaust valve under such circumstances.
The effects of different base oils on diesel engine timing chain wear were investigated in, “Investigation of Lubrication Effect on a Diesel Engine Timing Chain Wear,” Polat, Ozay, M. Sc. Thesis Istanbul Technical University Institute of Science and Technology (January 2008). This thesis concluded that the selection of base oil could influence timing chain wear in diesel engines.
Timing chain wear in light-duty diesel engines is due to a variety of factors one of which is the contribution of soot to abrasive wear. Li, Shoutian, et al., “Wear in Cummins M-11/EGR Test Engines,” Society of Automotive Engineers, Inc. (2001), paper no. 2002-01-1672. This article mentions that in engines with an exhaust gas recirculation (EGR) system, soot caused abrasive wear on liners, crossheads and top ring faces. The article also mentions that the main focus of soot-induced wear in non-EGR diesel engines has been on roller pin wear in the GM 6.2 L engine and crosshead wear in the Cummins M-11.
Chain elongation in gasoline engines is typically the result of roller pin wear. As a result, prior art methods for addressing timing chain stretch typically focus on use and selection of anti-wear agents. With the use of TGDi engines, soot is now a by-product of gasoline engine combustion and thus chain elongation due to such soot production may occur in such engines.
Lubricants currently used in gasoline engines to reduce timing chain stretch contain antiwear agents as it is thought that these additives are able reduce the timing chain wear. However, as demonstrated in the examples of the present application, certain typical anti-wear agents worsened timing chain stretch. In order to overcome the wear problem that results in timing chain stretch, a solution for reducing the rolling and sliding friction forces that cause roller pin wear is sought.
In some cases, dispersants and dispersant viscosity index improvers have been used to address wear problems. For example, U.S. Pat. No. 7,572,200 B2 discloses a chain drive system that employs a lubricant designed to coat the sliding parts of the system, including the chain and sprocket, with a thin hard carbon coating film having a hydrogen content of 10 atomic percent or less to reduce the amount of friction and wear on the chain drive system.
U.S. Pat. No. 8,771,119 B2 discloses a lubricating composition for a chain which comprises 80-95% by mass of a lubricant which is liquid at room temperature and 5-20% by mass of a wax that is a solid at room temperature. The addition of the wax is said to provide better abrasion resistance and provide a chain with elongation resistance and a longer life.
U.S. Pat. No. 7,053,026 B2 discloses a method for lubricating a conveyor chain system. Conveyor chains may be exposed to high temperatures and usually require polyol ester based lubricants. This patent focuses on reducing chain wear and minimizing deposits on chain surfaces by using a mixture of mineral oil, poly(isobutylene) and polyol ester.
The foregoing references do not provide an adequate solution for minimizing timing chain stretch in internal combustion engines. For example, the proposed use of dispersants for this purpose has been found to provide inadequate protection against timing chain stretch. Thus, the present disclosure provides a method of employing calcium detergents and detergent combinations in order to provide greater reductions in chain elongation than is provided by combinations of conventional anti-wear agents or dispersants.